Electrolytic tin plating bath



Patented May 15, 1 951 ELECTROLYTIC TIN PLATING BATH William S. Allen, Pittsburgh, Pa., assignor to United States Steel Company, a corporation of New Jersey No Drawing. Application August 27, 1947, Serial No. 770,956

8 Claims.

This invention relates to electrolytic tin plating baths and to addition agents therefor.

In electrolytic processes for plating tin on steel, steel is immersed in a plating bath from which tin is electrically deposited on the surface. Subsequently the plated metal may be subjected to further treatment, such as heating, to melt the tin deposit and enhance its luster. The baths are ordinarily aqueous solutions of a tin salt, various acids or alkalis and various addition agents.

Baths with which the present invention is concerned consist of an aqueous solution of a stannous salt and acid, such as phenolsulionic acid, phenolsulfonic acid and a phenol, sulfuric acid, or fluoboric acid, besides the addition agent. Many materials have been used as addition agents in such baths, perhaps the most common being glue, beta-naphthol or sulfones, such as dihydroxy diphenyl sulfone. The action of addition agents is not fully understood, but satisfactory tin deposits cannot be produced in their absence.

There are several variables that must be controlled in plating processes, these including current density, bath temperature, agitation, and the tin and acid contents of the bath. It is desirable that the operable range of each variable be as broad as possible, and such ranges may be broadened by proper selection of the addition agent. Th interrelation of these variables is such that broadening the operable range of one ordinarily has a broadening effect on the others. For example, an addition agent that broadens the operable current density range over that obtained with the usual sulfone addition agent likewise broadens the operable temperature range at fixed current density. It is apparanet that broadening the operable ranges of the variables facilitates plating operations, since less precise control is needed.

Accordingly, the principal object of the present invention is to provide improved addition agents for electrolytic tin plating baths for broadening the range of current densities capable of producing satisfactory tin deposits.

A further object of the invention is to provide improved addition agents for electrolytic tin plating baths that in the concentrations employed are soluble throughout the range of temperatures encountered and are stable compounds, not subject to decomposition by hydrolysis.

A further object of the invention is to provide improved baths for electrolytic tin plating by use of which control of the operation may be less precise than with previous baths with which I am familiar.

A further object of the invention is to provide improved electrolytic tin plating baths capable of forming heavier tin deposits of good quality, that is, melting to a high luster, than other baths with which I am familiar.

The addition agents of the present invention consist essentially of the polymerized reaction product of an aromatic primary amine and an aliphatic ketone. Specifically the monomers of these agents may be formed by combining one mol of an amine of the group consisting of aniline and ortho, meta and para toluidine with two mols of a ketone oi the group consisting of acetone and methyl ethyl ketone. Each reaction yields a monomer which is a dihydroquinoline derivative.

In the case of aniline and acetone, the monomer is 2,2,4 trimethyl-l,2 dihydroquinoline, which has the structural formula:

Similarly, ortho, meta and para toluidine with acetone yield, respectively: 2,2,4,8 tetramethyl-l,2 dihydroquinoline 2,2,4,'7 tetramethyl-1,2 dihydroquinoline 2,2,4,6 tetramethy1-1,2 dihydroquinoline In the case of aniline and methyl ethyl ketone, the reaction product is 2 methyl-2,4 diethyl-1,2 dihydroquinoline, which has the structural formula:

CZHE

C C CH /CH3 H e a we Similarly ortho, meta and para toluidine with methyl ethyl ketone yield, respectively:

2,8 dimethyl-2,4 diethyl-1,2 dihydroquinoline 2,7 dimethyl-2A diethyl-l,2 dihydroquinoline 2,6 dimethyl-2,4 diethyl-LZ dihydroquinoline It is to be noted in each of the foregoing compounds that the term dihydro is used to signify the saturation of the double bond normally present in quinoline between the N and the adjacent C, despite the substitution of an aliphatic group for one of these hydrogens.

Each of the foregoing compounds is capable of polymerization in the presence of an acid. Polymers suitable as additional agents in accordance with the present invention conveniently may be prepared by mixing about four parts by weight of one of the aforementioned dihydroquinoline deisdeemed unnecessary to describe in detail processes for their preparation.

Either the unneutralized acid solution of the polymer or the basic polymer is suitable-as the addition agent. In some instances the basic polymer may be dissolved directly in the bath, although I usually prefer to dissolve the basic polymer first in H2804 and add the resulting solutionto the bath.

The addition agent is added to acid baths in amounts in the range of 0.20 to 7.00 grams of agent per liter of bath. The baths contain tin in the form of a stannous salt usually within the limits of 10 to 80 grams of metal per liter of so lution with baths for strip plating usually having a tin concentration in the lower portions of the range on account of agitation of. the bath on strip travel. The acid content varies with different acids but is always sufficient to produce a solution having a pH value of not more than 2. Phenol sulfonic acid baths contain from 35 to 220 grams of the acid per liter, or on the basis of H280; equivalent 6 to 40 grams per liter. To this may be added up to 10 grams per liter of a phenol. Either ordinary carbolic acid may be used or other phenolic compounds, such as o, in or p cresol, resorcinol, naphthol, etc. Sulfuric acid baths contain from to 40 grams of the acid per liter. Fluoborate baths contain from. to v grams of hydrofiuoboric acid per liter, usually with a slight excess of free boric acid. The operating temperature is usually in the neighborhood of i00to 110 F. and in the case of strip plating, a considerable degree of agitation is produced by movement of the strip through the bath. a a

For any particular polymer and anyparticular bath composition :andoperating conditions, theof current densities. The rate of depletion of the materialsis slower than with baths containing as addition agents sulfoxides or sulfones, the most satisfactory "previously" used agents ofwhich' I have knowledge. The agents are completely soluble in acid solution and therefore do not crystallize and separate on cooling of the solution. They are stable compounds and do not hydrolyze. Tin deposits made using these agents produce unusually lustrous coatings on subsequent melting.

To demonstrate the efiicacy of addition agents of the present invention, the following examples of baths containing these agents and the resulting current density ranges are cited:

Example -I Aphenol sulfonic acid bath was prepared consisting of an aqueous solution of:

109 grams/liter 65% phenol sulphonic acid 65 grams/liter stannous sulfate 2.0 'grams/liwr polymerized 2,2,4 trimethyl-L2 dihydroquinoline The bath analysis was:

' Grams/liter Free acid (HzSOi equivalent) 20: Tin (stannous) 35.: Agent 2.0

Ihe operating conditions were:

Temperature F.

Agitated The operable current density.rangewas-found to be 30 to 1150 amperes/sqit.

Example II Polymerized 224,8 tetramethyl-1,2 dihydroquinoline was substituted as the. addition agent in a bath otherwise identical to that of Example Example III Po1ymerized2,2,l,'7 tetramethyl-l,2 dihydro' quinoline'was substituted as-the addition agentin a bath otherwise-identical to that of Example I, theconcentration of agent being 0.7 5'gram/ liter I and operating -C0llditi0lls remaining the same:

Theoperable current densityrange was found to be-35 to at least-600 amperes/sq-ft:

Example iIV" sisting of an aqueoussolution oft- 109 grams/liter; 65% 'phenoltsulfonic acid 65 grams/ liter stannous sulfate 5 grams/liter phenol 0.25- gram/literpolymer-ized- 2,2,4 trimethyl- -l 2 dihydroquinoline I The bath analysis was? g g V 0 Grams/liter Free-acid *(H2SGr-equivalent) 20? Tin (stannous) 35 Pheno1 5 Agent'-i 0.25

Theioperating. conditions remainedas .in EX}? ampleI." The operable currentdensity range was found to be 30 to 74=0'amp"eres/sq.,ft;

.A phenol sulfonic acid bathwas prepared" con sisting of an aqueous solutionol 109 grams/liter. 65% phenol. 's ulf'onici acid I V 36 grains/liter stanneous sulfate 5 grains/liter phenol dihydr'oquinoline The operable current density range A phenol .sulfonic. acidbath. was .prepared com;

7 0.25"gram/litei polymerized 2,2,4 trimethyl lfif The operable current density range was found to be 30 to 1220 amperes/sq. ft.

Example VI A sulfate bath was prepared consisting of an aqueous solution of:

42 grams/liter sulfuric acid 81 grams/liter stannous sulfate 3.0 grams/liter polymerized 2,2,4 trimethyl-l,2

dihydroquinoline The bath analysis was:

Grams/liter Free acid 42 Tin (stannous) 45 Agent 3.0

The operating conditions were:

Temperature F 100 Agitated The operable current density range was found to be 75 to at least 600 amperes/sq. ft.

Example VII Sulfate baths were prepared consisting of aqueous solutions of:

20 grams/liter sulfuric acid 65 grams/liter stannous sulfate 3 to 4 grams/liter polymerized 224,6 tetramethyl-1,2 dihydroquinoline The bath analyses were:

Grams/liter Free acid 20 Tin (stannous) 35 Agent 3 to 4 The operating conditions were:

Temperature F 100 Agitated The operable current density ranges were found to be 50 to at'least 600 amperes/sq. ft.

Example VIII Fluoborate baths were prepared consisting of aqueous solutions of:

20 grams/liter hydrofluoboric acid 6 grams/liter boric acid 86 grams/liter stannous fluoborate 3.0 to 7.0 grams/liter polymerized 2,2,4 trlmethyl- 1,2 dihydroquinoline Agitated 6 The operable current density ranges were found to be 25 to at least 600. amperes/sqft.

Example IX A phenol sulfonic acid bath was prepared consisting of an aqueous solution of:

109 grams/liter 65% phenol sulfonic acid 65 grams/liter stannous sulfate 0.5 gram/liter polymerized 2 methyl-2,4 cliethyl- 1,2 dihydroquinoline The bath analysis was:

Grams/liter Free acid (H2804 equivalent) 20 Tin (stannous) 35 Agent 0.5

The operating conditions were:

Temperatureuu F" 100 Agitated The operable current density range was found to be 50 to at least 600 amperes/sq. ft.

In each of the foregoing examples, light refiectivity tests were employed to determine light reflectivity, the deposit is considered satisfactory.

It is seen from the examples that sulfonic acid baths, sulfate baths or fiuoboric baths containing addition agents of the present invention produce satisfactory coatings over wide ranges of current density. In each instance the current density range is considerably broader than that obtained with sulfoxides or sulfones as the addition agents. Operable ranges of other variables likewise are increased as hereinbefore pointed out.

While I have disclosed but certain specific compounds and ranges and cited but a limited number of examples of bath compositions, it is apparent that further modifications may arise. For example, the agents disclosed may be used in combination with each other. Therefore, I do not wish to be limited by the disclosure set forth, but only by the scope of the appended claims.

I claim:

1. An electrolytic tin plating bath consisting es sentially of an aqueous acid solution having a maximum pH value of 2, of 10 to grams per liter of tin in the form of a stannous salt of an acid of the group consisting of sulfuric acid, phenolsulfonic acid and hydrofluoboric acid and 0.2 to 7.0 grams per liter of an addition agent which is a polymer of a monomer of the group consisting of: 2,2,4 trimethyl-1,2 dihydroquinoline, 2,2,4,8 tetramethyl-1,2 dihydroquinoline, 2,2,4,7 tetramethyl-1,2 dihydroquinoline, 2,2,4,6 tetramethyl-1,2 dihydroquinoline, 2 methyl 2,4 diethyl-1,2 dihydroquinoline, 2,8 dimethyl 2,4 diethyl-l,2 dihydroquinoline, 2,7 dimethyl 2,4 di ethyl-1,2 dihydroquinoline, and 2,6 dimethyl 2,4 diethyl-1,2 dihydroquinoline.

2. An electrolytic tin plating bath consisting of an aqueous acid solution having a maximum pH value of 2, of 10 to 80 grams per liter of tin in the form of a stannous salt of an acid of the group consisting of sulfuric acid, phenolsulfonic acid and hydrofiuoboric acid and 0.2 to 7.0 grams per liter of a polymer of 2,2,4 trimethyl-1,2 dihydroquinoline.

3. An electrolytic tin plating bath consisting essentially of an aqueous acid solution having a maximunnpI-I. valuezof 2, of;; to: 80:grams;-v per liter of tin in; .the .form. lof a .stannousi salt of- .1

acid of the group .consistingof sulfuric acid,

phenolsulfonic acid and'hydrofiuoboric acid and i 0.2 to 7.0; gi arnsperylite'r.of:anlad.dlitionv agent,

the acidity being produced lby *thepresence, of an.

acid. of the group ,consisting otphenolsulfonic acid, phenolsulfo'nic acid and a phenol, sulfuric 1,2 dihydroquinoline, 2,2,4,8 tetramethyl-'1,2 dihydroquinoline, 2,2,4,7 tetramethyl-1,2 dihydroquinoline, 2,2,4,6 tetramethyl-1,2. dihydroquinov line, 2 methyl 2,4 diethyl -1,2 dihydroquinoline, 2,8 dimethyl 2,4 diethyl-1,2 dihydroquinoline; 2,7

dimethyl 2,4 damn-1,2 dihydroquinoline, and

2,6 dimethyl 2,4 methyl-1,2 "dihydroquinoline;

4. An electrolytic tin plating, bath consisting goz'lmethyl 2,4 d1ethy1-1,2 dlhydroquinohne, ,2,7.d1-

methyl 2,4 diethyl-1,2 dihydroquinoline, and 2,6

essentially of an aqueous solution of 10 ,110 80 grams per liter of tin initially in the form of stannous sulfate, to 220 grams per liter of phenol ,sulfonic ,acid; and 0.2 to 77.0. grams per liter of janiaddition agent" which I'is'a polymer of a monomer of the group 'con'sistingof 2,2,4 tri-""' 5. "An "electrolytic tin j plating jbath "consisting essentially"ofan aqueous solution of l 10' to' 80 gramsfiperj liter 'of'tin initiallyjini..tlie form ofstannous"sulfate,i35to220'"grams per liter of phenol'sulfonic acid; a phenol inan amount up 4(j,$,- REFERENCES CITED to 10 grams per liter, and.0.2 to 7 grams perlit'er' of an"'addition"agient Which'is a poly'r'neror" a mononieri'of'th, group consisti'rig'jof f2,2,4 .tri-' methyl-1,2 dii'iydro'quinoline; '2,2';4,8' tetramethyh 1,2 dihydroquinoline, 2,2,4,7 tetramethyl-1,2 'di quinolin'e 2 methyl"2,4. diethyl-"LZ dihydro'quino-i line;"2;8'dimethyl2,4 diethyl 1,2 dihydroquinoline;1 2,7 dimethyl""2,4- methyl-1,2 .dihy'clroquinoliiie and 2,6 dimethyl 2,4 diethyl1,2 dihydroquinolin'e. 6. An, electrolytic. tin plating bath consisting essentially-of 'ani-aqueous'solution"of 10 to 80 grams-per liter "of tininitiallyin" the form 'of stannoussulfatef 35' to 220-grams 'perliter of "i phenol sulfonic' acid; and 0.2 to"4 grarnsper liter" of an addition agent which" is a polymerof' a monomer of the'group'consistingofr 2,2,4tri= methyl-1,2 dihydr'oquinoline; ,2,2,4,8,.tetramethyl-' 1,2 dihydroquinoline, 2,2,4,7 tetramethyl-l,2 dihydroquinoline, 224,6 tetramethyl-1,2 dihydro quinoline, 2 methyl 2,4 diethyl-1,2 dihydroquino- I ,1line, 2,8 'dimethyl 2,4 diethy1-1,2 'dihyolroquinoline, 2,7 dimeth'yl 2,4 diethyl-1,2'dihydroquinoline, and 2,6 dimethyl 2,4 diethyl1,2 dih'ydroquinoline.

'7. An electrolytic tin plating bathflconsisting; claessentially of an aqueous solution of 10 to 80 grams per liter of tin initially in the form of stannous sulfate, 6 to 40 grams per *liter of *sulfuric acid, and. 0.2 to '7 grams per liter of an addition agent which is .a polymer'of a monomer of lfjilithe group consisting of: 2,2,4 trimethyl-1,2 dihydroquinoline, 2,2,4,8 tetramethyle'l,2 ,dih'ydroquinoline, 2,2,4,7 tetramethyl-LZ dihydroquinoline, 2,2,4,6 tetramethyl-1,2 ,dihydroquinoline, 2

methyl 2,4 diethyl-l,2 -dihydroquinoline, 2,8jdidimethyl 2,4 cliethyl-1,2 dihydroquinoline.

8'. An electrolytic tin plating bath consisting essentially of an aqueous solution of 10 to ,Lgiams per liter of tin initially in the form of stannous fiuoborate, 5 to 30 grams per liter of hydrofluoboric acid, and 0.2 to 7 grams per liter 01' an addition agent which is a polymer of a monomer of the group consisting of 2,2,4 tri- :methyl-LZ dihydroquinoline, 2,248 tetramethyl- 1,2 dihyclroquinoline, 2,2,4}? tetramethyl-1,2 clihyolroquinoline, 2,2,4,6 tetramethyl-l,2 dihydroquinoline, 2 methyl 2,4 diethyl-1,2 dihydroquinoline, 2,8 dimethyl 2,4 diethyl-LZ dihydroquino- 35;;line, 2,7 dimethyl 2,4 diethyl-.1,2 clihydroquinoline, and 2,6 dimethyl 2,4 diethyl-1,2 dihydroquinoline. WILLIAM S. ALLEN'.

The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,824,100 Schlotter Sept. 22, 1931 2,370,986 Nachtman Mar. 6, 1945 OTHER REFERENCES Transactions of the American Electrochemical Society, vol. 33 (1918), pages -168 (an article:

by Kern) Transactions of the American Electrochemical Society, *V0l;"38 (1920), page 148 (an article by 

1. AN ELECTROLYTIC TIN PLATING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS ACID SOLUTION HAVING A MAXIMUM PH VALUE OF 2, OF 10 TO 80 GRAMS PER LITER OF TIN IN THE FORM OF A STANNOUS SALT OF AN ACID OF THE GROUP CONSISTING OF SULFURIC ACID, PHENOLSULFONIC ACID AND HYDROFLUOBORIC ACID AND 0.2 TO 7.0 GRAM PER LETER OF AN ADDITION AGENT WHICH IS A POLYMER OF A MONOMER OF THE GROUP CONSISTING OF: 2,24 TRIMETHYL-1,2 DIHYDROQUINOLINE, 2,2,4,8 TETRAMETHYL-1,2 DIHYDROQUINOLINE, 2,2,4,7 TETRAMETHYL-1,2 DIHYDROQUINOLINE, 2,2,4,6 TETRAMETHYL-1,2 KIHYDROQUINOLINE, 2 METHYL 2,4 DIETHYL-1,2 DIHYDROQUINOLINE, 2,8 DIMETHYL 2,4 DIETHYL-1,2 DIHYDROQUINOLINE, 2,7 DIMETHYL 2,4 DIETHYL-1,2 DIHYDROQUINOLINE, AND 2,6 DIMETHYL 2,4 DIETHYL-1-2 DIHYDROQUINOLINE. 